Magnetic disk drive systems typically include one or more sliders that contain read and write heads that are used to read and write data in thin film magnetic media on one of more rotating disks. The sliders are mounted on a movable actuator that is positioned by a servo system over the rotating disk using a VCM. The information in the thin film magnetic material on each disk surface is organized into concentric circular tracks that are further divided into angularly spaced servo sectors. Each sector has servo information permanently recorded therein as a part of the manufacturing process. The user data in the sector follows the servo information. The servo information typically includes track and sector location information, automatic gain control (AGC) fields, timing sync signals, and servo bursts. The servo bursts are arranged in specially designed patterns to generate a read head signal that gives fractional track centerline information from which the drive's controller can determine where the read head is in relation to the center of the track. One typical servo burst pattern includes four bursts referred to as A, B, C and D and is called a quad-burst pattern. FIG. 1A is a block diagram illustrating the allocated linear positioning of fields in a quad burst servo pattern according to the prior art. The actual cross-track positions of the A, B, C and D bursts are arranged in a repeating pattern that provides the fractional track centerline information. Depending upon the SNR requirements, the burst pattern may be written at lower linear densities than the AGC field. The cross-track widths of the servo bursts can be equal to one-track width, but other widths can be used such as two-thirds or one-half of a track width. An alternate servo system using a “null pattern” using P and Q bursts is illustrated in FIG. 1B. FIG. 1C illustrates two adjacent tracks that have a servo burst pattern written at one-half of a track width. The “+” and “−” symbols represent the two magnetic orientations that have been previously written in the thin film magnetic material on the disk during the manufacturing process.
Seamless servo writing techniques use independent positioning of the bursts and AGC fields during servo writing process such that different regions of a servo pattern are written in different steps or passes. Seamless and seamed servo burst patterns using P and Q bursts have also been developed.
FIG. 1D illustrates two adjacent tracks that have a servo burst pattern written at one-half of a track width with both AGC and servo burst fields written in a seamed manner in four passes which are represented by the circled numbers 1-4. FIG. 1E illustrates the write gate pattern for the four phases of the writing process for two tracks k and k+1 illustrated in FIG. 1D. In each of the four phases the AGC, P and Q are written as one-half track width.
FIGS. 1F and 1G illustrate the four phases of writing a seamed AGC with seamless P and Q bursts on two adjacent tracks. In each of the four phases the AGC is written, but the P burst is only written on the first and third phase and the Q burst is only written on the second and fourth phase. In this example and the following ones, the two track pattern is repeated for the entire set of tracks.
FIGS. 1H and 1I illustrate the four phases of writing a seamless AGC with seamed P and Q bursts on two adjacent tracks. In each of the four phases the P and Q bursts are written, but the AGC is only written on the first and third phase. FIGS. 1J and 1K illustrate the four phases of writing a seamed AGC with seamless P and Q bursts on two adjacent tracks. The AGC is only written on the first and third phase. The P bursts are written on the first and third phase, and the Q bursts are written on the second and fourth phase.
The servo fields define the track width and the pitch. Ideally each track will have the same width and the track pitch will be constant as illustrated in FIG. 1L. The width of the track is defined during the servo writing process. FIG. 1M illustrates a problem case in which the track pitch in one track (k+1) has a narrower width than the other tracks. This is called track squeeze.
The signals generated in the read head by the servo fields are processed by the drive's electronics that include a pre-amplifier and a read/write channel. In US patent application 20110188147 by Cho, et al. (Aug. 4, 2011) a method of adjusting gain of a variable gain amplifier (VGA) of a read/write channel is described. The VGA amplifies a signal provided by the pre-amplifier based on a gain control signal of the AGC.
US patent application 20120014011 by Wilson (Jan. 19, 2012) describes a magnetic recording system with a read channel that includes a variable gain amplifier (VGA) and continuous-time filter (CTF), and an analog-to-digital converter (ADC) 122 connected in series to produce an input signal for a digital signal processing (DSP) block. The input signal for the read channel comes from the preamp which includes a loopback channel that allows a user to characterize frequency response at selected frequencies, and to derive a correction factor to remove gain changes. The loopback channel supports low-nm fly-height measurement. The loopback channel also allows write data-to-read data timing to be measured, in support of bit-patterned media (BPM).
U.S. Pat. No. 7,551,390 to Wang, et al. (Jun. 23, 2009) describes a method to characterize misaligned servo wedges. The servo controller is used to characterize misaligned servo wedges by implementing operations including: commanding the head to track follow on a track; measuring wedge-to-wedge time (WTWT) values corresponding to time intervals between identified servo wedges; calculating wedge-to-wedge time (WTWT) variations for the measured WTWT values; and characterizing the calculated WTWT variations, wherein, characterizing the calculated WTWT variations for the track includes utilizing a WTWT variation modeling function to model the WTWT variations.
In U.S. Pat. No. 7,663,835 (Feb. 16, 2010) Yu, et al. describe a method of measuring track squeeze based on ABCD burst readback profile. The method is based on using head read-out signal for servo bursts (A, B, C, D) to estimate the servo written-in errors or track squeeze by comparing measured burst readings to nominal values. This method requires an accurate burst readback profile. The voltage readback will vary based on the amount of phase misalignment and thereby introduce error in burst readback measurement.
Track misalignment refers to finite circumferential shift in servo sector locations between adjacent tracks. Misalignment arises from disturbance in servo writing process (SSW) which results in deviation from ideal servo sector location. FIGS. 2A-D illustrate a track misalignment problem in the prior art in various combinations of seamed and seamless AGC and P-Q burst servo formats.
Track misalignment causes magnetic signal loss, which is addressed by the present invention. As illustrated in FIG. 3, for the same physical misalignment, signal loss is different for 1T versus 2T servo patterns (where T is related to the linear bit density of the signal).
As illustrated in FIGS. 4A-B, track misalignment in seamless patterns is further complicated where AGC, P and Q bursts can have different misalignment. This results in further distortion in burst profile where one burst has same misalignment as AGC while other burst does not. VGA gain adjustment follows AGC misalignment.